The etiology of the lateralization of sensory, motor, and cognitive function is a fundamental problem in biology and neurosciences. Whereas man possesses a brain with two cerebral hemispheres that are roughly mirror images of each other, humans interact with their environments with marked asymmetries of function. Hereditary factors have been frequently implicated as one of the possible causes of differences between individuals in lateralization. Yet experimental studies of handedness in genetically uniform laboratory mice, as well as analyses of such data in human monozygous and dizygous twins, lead to the strong inference that genetic alleles are themselves left-right indifferent. Recent experimental studies using mice indicate that the degree of lateralization, rather than the direction of the sense of asymmetry, may be genetically regulated. This was observed as a difference between the sexes in the strength of lateralization, and it is not known to what degree autosomal genetic variants lead to differences in the degree of lateralization. A selective breeding program has been implemented using a foundation population obtained from crossing six distantly related inbred strains and two subspecies of mouse. In one line directional selection is being practiced for strong lateralization, and in the other, weak lateralization. The speed of response to selection may indicate the magnitude of genetic effects on lateralization itself, and the lines developed may prove useful in understanding possible correlates and substrates of variable lateralization, such as, for example, directional confusion exhibited in humans with dyslexia. Additional studies are concerned with asymmetries considered two at a time, the development of lateralization, effects of social learning or culture on the diffusion of asymmetries, and theoretical investigations of complex inherited systems. The laboratory mouse is the animal of choice in these experiments.